1. Field of the Invention
The present invention relates generally to the fields of pharmacology and drug delivery. More specifically, the present invention relates to a method of using carbon dioxide gas to increase pulmonary deposition of an aerosolized drug during inhalation therapy.
2. Description of the Related Art
Small particle liposome aerosol treatment consists of lipid-soluble or water-soluble anti-cancer drugs incorporated into liposomes, which are administered from aqueous dispersions in a jet nebulizer (see U.S. Pat. No. 5,049,388). Aerosols of 1-3 μm mass median aerodynamic diameter, generated upon nebulization, enable targeted delivery onto surfaces of the respiratory tract. The deposited liposomes subsequently release drug locally within the lung or into the blood circulation with delivery to extra-pulmonary tissue.
If the drug is lipid soluble, it will associate with the lipid molecules in a manner specific to the lipid employed, the anti-cancer drug employed and possibly it may be modified further by various soluble constituents which may be included in the suspending aqueous medium. Such soluble constituents may include buffering salts and possibly inositol to enhance the synthesis and secretion of surfactant phospholipid in lung tissue and to minimize respiratory distress already present or that which might result from the aerosol treatment (7).
If the drug is water soluble, it may be incorporated by appropriate procedures in aqueous vesicles that exit in concentric spaces between lipid bilayers (lamellae) of the multilamellar liposome. Unilamellar liposomes may be prepared; however, their capacity to entrap either lipid-soluble or water-soluble drugs is diminished since entrapment is restricted to one central vesicle. Aerosol water droplets may contain one or more drug-liposomes. Moreover, it is also possible to incorporate more than one drug in a aerosol liposome treatment, either by mixing different drug-containing liposomes, or by using liposomes wherein the drugs have been combined and incorporated together into liposomes.
Nebulization shears liposomes to sizes readily discharged from the nozzle of the nebulizer. Liposomes up to several microns in diameter are typically sheared to diameters of less than 500 nm, and may be considerably smaller than that depending on the operating characteristics of the nebulizer and other variables. Shearing of water-soluble drugs contained in liposomes will release appreciable amounts of the water soluble compound, perhaps 50 percent. This is not a contraindication to their use, but it means that two forms of the drug preparation is administered, and the effect includes the therapeutic effect that would be produced by both forms if either form has been given alone. Many other details of liposome aerosol treatment are described in U.S. Pat. No. 5,049,388.
In general, the underlying objective of inhalation therapy is the topical delivery of aerosolized particles of pharmaceutical drugs into the central airways and to peripheral regions of the respiratory tract. However, the deposition fraction of the inhaled particles even for the optimal size range of 1-2 μm mass median aerodynamic diameter is only approximately 20%. Pulmonary deposition of inhaled aerosols is influenced significantly by particle size, hygroscopic properties and airway geometry (1,2). The breathing pattern is also an important variable that determines the deposition pattern of inhaled particles (1,2).
Specifically, breath holding markedly increases pulmonary deposition due to increased residence time of particles within the lung. This allows a longer period for gravity sedimentation to occur especially in the small peripheral airways and to ensure that the aqueous particles can equilibrate fully in the near 100% humidity and reach their maximum size, which further enhances their deposition (1,2). Computer simulations demonstrate that a thirty-second breath holding maneuver in humans can increase the deposition fraction 3.2 times. The physiological principle of this effect is due to increased particle intake upon deep inspiration in which the inhaled volume may be as much as 8-fold higher than the amount inhaled with basal tidal breathing. This larger volume of tidal breathing leads to penetration of particles to the furthest recesses of the lung where airway diameters are smallest, and thus deposition due to gravity and maximum particle size occurs with greatest efficiency.
By extension of this physiological property, direct utilization of factors which could increase the volume of inspired air (containing aerosol particles) would subsequently markedly increase the deposited fraction in the central airway and to an even greater extent in the peripheral lung. Carbon dioxide (CO2) is the most important natural regulator of respiration. Carbon dioxide diffuses freely from the tissues into the blood according to the existing pressure gradient. Increased levels of carbon dioxide in the blood readily diffuse into the cerebrospinal fluid where there is conversion into HCO3− and H+. Central chemoreceptors on the ventral surface of the medulla respond to increased H+ in the CSF and cause a compensatory increase in ventilation (rate and tidal volume).
Investigators have utilized carbon dioxide inhalation to manipulate ventilation in experimental animals and humans. Inhalation of 5% carbon dioxide causes as much as 192% increase in tidal volume (3). This increase is rapid and reaches a sustained plateau throughout the duration of exposure (4). Once the carbon dioxide exposure ceases, the changes in ventilation reverse within minutes to basal level (4). Similarly, inhalation of 5% carbon dioxide by humans results in a 3-fold increase in the minute volume (5). Inhalation of 5% or 7.5% of carbon dioxide by normal humans for two minutes resulted in increases in frequency of breathing by 6.7% and 19%, respectively, and increases in tidal volumes by 31% and 52%, respectively, so that minute volumes were increased by 34% and 75%, respectively (6). Longer exposures to these concentrations would have produced even greater responses (5).
Camptothecin analogues and taxanes are chemical agents currently being developed as chemotherapeutic agents (21, 26). The anticancer drugs, paclitaxel (PTX) and different camptothecin (CPT) derivatives are clinically active in the treatment of a variety of human tumors, including lung cancer. These drugs show beneficial results in clinical trails when used as single agents or in combination with other drugs (21). These drugs are given systemically by oral or intravenous routes of administration; the most effective route for paclitaxel is continuous intravenous infusion (22,24) whereas lipophilic congeners of camptothecin administrated orally prove most effective. The development of toxic side effects is often a major limitation in such therapeutic regimens. Several subcutaneous human cancer xenografts in nude mice (23) and in experimental murine pulmonary metastasis (6) have been successfully treated using liposomal formulations of camptothecin and 9-nitrocamptothecin (9NC) administered by the aerosol route as an alternative method of therapy. Pharmacokinetic studies in mice with camptothecin showed that inhalation of liposomal camptothecin produced substantial drug levels in the lungs and other organs, which cleared rapidly after cessation of aerosol delivery (17). In spite of these levels, aerosol delivery systems are generally only 15-20% efficient in drug deposition (29, 30); thus increasing pulmonary deposition would be advantageous.
Using these systemic routes of drug delivery, a certain amount of drug egresses from the blood stream and localizes in the respiratory tissue, but lungs are not the main organs for drug deposition. The utilization of conventional liposomes are carriers for these drugs does not improve the pulmonary deposition of drugs administered by commonly used systemic routes ( 11,27). Nebulization is a very effective route for target drug delivery to the respiratory tract (17); e.g., camptothecin. Dogs with spontaneously arising primary and metastatic lung tumors have been successfully treated when new formulations of doxorubicin and PTX are delivered via aerosolization (16). However in these instances, aerosols were generated using normal air.
Gene delivery to different tissues has been accomplished using both viral and nonviral vectors. Although the use of nonviral vectors avoids the immunogenic response associated with viral vectors, nonviral vectors, such as cationic lipids and polycationic polymers, have not been associated generally with the high levels of gene expression characteristic of viral vectors. However, polyethyleneimine (PEI), a cationic polymer, is effective both in tissue culture and in vivo (36). The protonable nitrogen on every third nitrogen provides polyethyleneimine with a huge buffering capacity. Polyethyleneimine can effectively traffic DNA to the nucleus (37) and protect DNA against DNAse degradation (36). Both linear and branched forms of polyethyleneimine have been shown to produce high levels of transgene expression in various tissues such as lung, brain, and kidney (39-41). Polyethyleneimine has also been used to efficiently deliver DNA to tumors in vivo (42).
Aerosol delivery is a noninvasive way to deliver genes of interest to the lungs and could potentially be used to treat diseases such as lung cancer and cystic fibrosis. However, the levels of transgene expression have not been very high due, in some cases, to loss of DNA viability during nebulization (43). PEI can protect the DNA during nebulization (44) and can result in higher levels of transfection in the lung than most of the other cationic lipids tested (44,45). PEI-mediated transfection is also resistant to inhibition by lung surfactants (46).
Increased efficiency of drug deposition to the respiratory tract by the inhalation route is achieved by several ways: 1) changing the concentration of drug in the formulation used for aerosolization (31); 2) using more efficient types of nebulizers (32); 3) increasing the duration of treatment; or 4) changing the breathing patterns (4). As previously stated, carbon dioxide is a natural modulator of respiration. The inhalation of air containing low concentrations of CO2 (from about 3-7%) caused similar changes in breathing patterns and was tolerated well (13, 6). No difference in breathing patterns was observed between inhalation of 5% CO2-in-air and moderate physical exercise in man (32). Similar effects of 5% CO2-in-air may be obtained in man using aerosol treatment. Thus utilization of CO2-enriched air for nebulization as a modulator of inhalation therapy can result in more effective pulmonary delivery of chemotherapeutic agents.
The prior art is deficient in the lack of a means of enhancing the pulmonary deposition of an aerosolized drug during inhalation therapy. The present invention fulfills this longstanding need and desire in the art.